The present invention relates to a subscriber line monitoring circuit capable of testing subscriber line capacity.
In general, there are many instances where a subscriber line and a telephone set connected to the subscriber line can be equivalently considered as a capacitor alone. The capacity of the equivalent capacitor is referred to the subscriber line capacity. In such a case, a conventional system has adopted the following test sequence for carrying out a test of the subscriber line capacity. Namely, first step is to connect a subscriber line to a test circuit comprising e.g. relay circuitry etc. by means of a suitable switch assembly. Second step is to apply a power supply voltage to the subscriber line from the side of the test circuit to charge the equivalent capacitor. Third step is to form a closed circuit including the equivalent capacitor and a reference capacitor provided in the test circuit to discharge an electric charge stored in the equivalent capacitor into the reference capacitor. In this instance, since the total charge of the both capacitors is constant, a voltage appearing across the equivalent capacitor is determined by the relationship proportional to a voltage-divisional ratio of capacities of the both capacitors and to the power supply voltage. Accordingly, when the values of the power supply voltage and the capacity of the reference capacitor are given, the subscriber line capacity is evaluated based on the voltage appearing across the equivalent capacitor, thus enabling a capacitive test of the subscriber line.
However, such a conventional system cannot be applied to a telephone set including a diode bridge in its subscriber line interface for the reason stated below. Namely, where the above-mentioned test sequence is implemented to such a telephone set, a backward voltage occurs when the third step is executed in order to form the closed circuit. This backward voltage is applied to the diode bridge, failing to form the closed circuit for discharging the electric charge stored in the equivalent capacitor into the reference capacitor. For this reason, the above-mentioned conventional monitoring system cannot be applied to a capacitive test of the subscriber line to which a telephone set including a diode bridge in its interface is connected.